Ultrathin, flexible, and sandwiched structure composite polymer electrolyte membrane for solid-state lithium batteries

Solid-state composite polymer electrolytes (CPEs) are promising for solving the safety problem of lithium metal batteries. However, the CPEs with comparable thickness (10 μm), improved ion conductivities and excellent mechanical strength are great challenges. In the present study, ultrathin (9.6 μm), flexible, mechanically strong, and sandwiched structure solid-state electrolytes (SSEs) are fabricated by a tape casting process, involving the 75 wt % LLTO/PVDF-CPE (LLTO-75) layer designed as intermediate layer sandwiched by two 15 wt % LLTO/PVDF-CPE (LLTO-15) layers. The LLTO-15 is a soft, stable layer on upper and lower sides that could create excellent interfacial contact with electrodes, and LLTO-75 possesses physical rigidity that could inhibit the Li dendrites growth. The balancing between the excellent interfacial contact of LLTO-15 and the inhibition of lithium dendrites growth of LLTO-75 gives sandwiched SSE a high ionic conductivity (4.7 × 10-4 S cm-1) at room temperature, an excellent mechanical strength (7.2 MPa), and uniform Li plating/stripping. When coupled with LiFePO4 and Li metal, the battery assembled with sandwiched SSE maintains 91.7% of its capacity and 99.7% of the Coulombic efficiency, after 1000 cycles. Our study suggests that the proposed sandwiched structure SSE is one of the alternatives for high performance solid-state LMBs.